Calcareous nannofossil assemblage from the Setana Formation, Southwestern Hokkaido, Japan (Part 1)

Abstract

The Setana Formation, consists mainly of loose, coarse sand, widely developes in the Southwestern part of Hokkaido. It has long been considered as the representative marine Pliocene stratum in Hokkaido. Two types of calcareous nannofossil assemblages are discriminated in this Setana Formation. One is predominated by Coccolithus pelagicus and the other by Gephyrocapsa spp. Ecological consideration of recent coccolithophorid strongly suggests that the former is indicative of cold-water flora and the latter is temperate. UOZUMI'S suggestion (1962) that the Setana Molluscan Fauna was under the influence of both cold and warm current, is well coincident with the present consideration. Based on the occurrence of calcareous nannofossils as various morphotypes of Gephyrocapsa and P. lacunosa, the Setana Formation is obviously correlatable with the coccolith subzone CN14a (0.9-0.3 Ma) of OKADA and BUKRY (1980). Age of the Setana Formation therefore is Early to Middle Pleistocene.